1. Field of the Invention
This invention relates to a process for producing a high concentration coal-water slurry. More particularly it relates to a process for producing a coal-water slurry suitable for reducing its production cost.
2. Description of the Prior Art
Recently, using coal in place of expensive petroleum has become popular. However, coal in the form of a solid fuel is difficult to handle and also the cost of its transportation is great. Thus the development of techniques of converting coal into a slurry to make it possible to handle coal in the form of a fluid has been energetically carried out.
As one of the techniques, a process of COM (Coal and Oil Mixture) obtained by mixing coal with heavy oil has been known. This process, however, is directed to a mixture of coal with heavy oil in a ratio by weight of about 1:1; hence it cannot be regarded as a completely oil-free fuel and also its merit in cost is small. Further, a mixture of coal with methanol, the so-called methacoal, has been also known, but since expensive methanol is used therein, the mixture is also expensive so that it has not yet reached a stage of practical use.
On the other hand, CWM (Coal and Water Mixture) which is a mixture of coal with water is fully practical also in cost; hence it recently has been most noted. CWM, however, has a problem that if the water content therein is high, its heat efficiency at the time of combustion decrease, and contrarily if it is low, the viscosity of CWM rises to increase the pressure loss at the time of transportation. Further, since CWM consists of coal particles and water, there is a problem of storage that coal particles settle with lapse of time and separate from water. In order to overcome these problems, there has been made an attempt of adjusting the particle diameter of coal particles to thereby produce a CWM having a low viscosity and a good stability.
In order to produce a CWM slurry having a high coal concentration, a low viscosity and a good stability, it is said to be preferable to mill coal so as to give such a particle diameter distribution that the packing fraction of coal may be made as high as possible. As one of the processes for milling coal so as to give such a particle diameter distribution, a high concentration wet milling process has generally been known wherein coal is milled in a high concentration of 60-80% (% by weight; this applies to the following descriptions). However, when the coal concentration becomes so high, the viscosity of slurry also becomes high, which inevitably results in a problem of reduction in the milling efficiency, i.e. increase in the power consumed in the mill. Further, according to such a high concentration wet milling process, it is necessary for promoting the milling to add an additive such as surfactant (dispersing agent), but since the amount necessary amounts to about 1% of the weight of coal used, its influence upon the production cost of CWM cannot be neglected.
The object of the present invention is to provide a process for producing a CWM having overcome the above-mentioned drawbacks of the prior art and having a low viscosity and a good stability even in a high coal concentration without any increase in the production cost.